What came first, Fourier's Law, or the definition of heat capacity?
Warning -- assumptions made in the following.
Fourier's Wiki page mentions that
In 1822 Fourier published his work on heat flow in Théorie analytique de la chaleur (The Analytical Theory of Heat) Which one could interpret as the first publication of what later became named "Fourier's Law."
Whereas the Wiki page on heat capacity states
In a previous theory of heat common in the early modern period, heat was thought to be a measurement of an invisible fluid, known as the caloric. Bodies were capable of holding a certain amount of this fluid, hence the term heat capacity, named and first investigated by Scottish chemist Joseph Black in the 1750s.
Thus one can assume (making an ass of you and me) or assert (see the title-text) that heat capacity was defined first.